Notices
Results 1 to 4 of 4

Thread: Monoprotic acid strength

  1. #1 Monoprotic acid strength 
    Forum Freshman
    Join Date
    Aug 2006
    Location
    Australia
    Posts
    20
    Are monoprotic acids generally more acidic than di/triprotic acids at the same level of ionisation?


    Reply With Quote  
     

  2.  
     

  3. #2  
    Moderator Moderator AlexP's Avatar
    Join Date
    Jul 2006
    Location
    NY
    Posts
    1,838
    i don't know this for a fact, but i'd say no. acidity, or the strength of an acid, is determined by the amount of ionization that takes place. So, if it ionizes at the same level, like you said, i'd think that it technically has the same acidity.


    "There is a kind of lazy pleasure in useless and out-of-the-way erudition." -Jorge Luis Borges
    Reply With Quote  
     

  4. #3  
    Forum Freshman
    Join Date
    Aug 2006
    Location
    Australia
    Posts
    20
    I've talked to ex head of our chemistry dept. and he has told me that if you have to stong acids i.e. ones that fully disociate in solution and one is monoprotic (say HCl) and the other diprotic (say H2SO4) then the diprotic acid will have a lower pH. In other words be a stronger acid.

    There are more H3O+ (hydronium ions) per volume.
    Reply With Quote  
     

  5. #4  
    Forum Sophomore Matt Lacey's Avatar
    Join Date
    Sep 2006
    Location
    Southampton, England
    Posts
    119
    I'm gonna say no as well.

    Polyprotic acids don't dissociate totally in solution, each proton leaves one at a time, like...

    H<sub>2</sub>SO<sub>4</sub> <=> HSO<sub>4</sub><sup>-</sup> + H<sup>+</sup> <=> SO<sub>4</sub><sup>-</sup> + 2H<sup>+</sup>

    Acidity is measured by pKa which is a logarithmic scale. Because sulfuric acid is a diprotic acid, it has 2 pKa values, one for the first deprotonation and one for the second - these are -3.00 and 1.99 respectively. Compare this with -8 for HCl. Also, the pKa for H<sub>3</sub>O<sup>+</sup> is -1.76 which makes it a stronger acid than HSO<sub>4</sub><sup>-</sup>. My guess is that at moderate concentrations HCl will have a lower pH than sulfuric acid.
    Reply With Quote  
     

Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •